Modern spacecraft may include heat generating devices such as power amplifiers, oscillators, batteries, recording sensors and the like which cumulatively produce significant heat. In addition, spacecraft may include surfaces which face the sun or other radiant heat sources for long periods of time.
The heat generated within the body of the spacecraft, the heat received from the sun or other radiant source, or the combination of the two, tends to raise the temperature of the spacecraft, particularly that portion facing the radiant heat source. As the temperature rises, the spacecraft tends to lose heat by radiation into space. At some temperature of the spacecraft, heat loss due to thermal radiation will equal the heat gain, and the temperature will stabilize. However, some portions of the spacecraft may reach a temperature higher than desired. Such a temperature might be one at which a piece of equipment fails or exceeds its specified limits.
It is desirable to reduce the heat gain from the radiant sources. This may be particularly important if the radiant source is a hostile laser. U.S. Pat. No. 4,899,810, issued Feb. 13, 1990 in the name of Fredley, describes a scheme for cooling spacecraft by the use of circulating coolant fluid and heat rejection panels, which allows portions of the structure to be cooled by transferring heat to the panels. Under many conditions, it will not be possible to use such a cooling system. Another prior art technique surrounds portions of the spacecraft with a barrier or blanket, which tends to reflect light and heat energy. A conventional thermal protective barrier or blanket might include a multilayer structure, i.e., ten layers of aluminized clear or transparent dielectric sheet such as Kapton sheet. Kapton is a trademark for polyimide film manufactured by E. I. DuPont de Nemours Company. The innermost layer of the Kapton sheet is reinforced with a glass fiber mesh adhesively bonded to its outer surface. The aluminization is electrically conductive, and acts as a reflector which tends to reflect heat and visible light away from the spacecraft body. The outer Kapton layer has a transparent conductive coating on its outer surface to reduce electrostatic discharge which might occur due to the accumulation of charge on the outer surface of the Kapton. The transparent, electrically conductive coating may be indium-tin oxide (ITO) or germanium. The eight layers of Kapton lying between the innermost and outermost sheets are embossed to reduce large areas of contact which might provide low thermal resistance paths between inner and outer layers. Such a barrier has a solar absorption (.alpha..sub.s) of about 0.43, and a normal (orthogonal) emissivity (.epsilon.) of about 0.77 relative to a black-body radiator. The transmittance through the blanket is zero at any of the infrared (IR) bands listed in Table I. The absorption (.alpha.) and reflectance (.rho.) for three ranges of infrared radiation (in micrometers) are listed in Table I.
TABLE I ______________________________________ .alpha. .rho. ______________________________________ 3-5 .mu.m &gt;0.20 &lt;0.80 5-10 .mu.m &gt;0.80 &lt;0.20 10-25 .mu.m &gt;0.70 &lt;0.30 ______________________________________
In general, the blanket should ideally have as high a reflectance (.rho.) as possible to reflect thermal and solar energy, and as low an absorption (.alpha.) as possible to prevent a temperature rise of the blanket, which might be communicated to the spacecraft body. The emissivity (.epsilon.) should be high, so that temperature rises due to absorption of energy are controlled by re-radiation into space. Other considerations must also be taken into account, such as the transmittance of energy, IR and visible light through the blanket, which should ideally be zero. Also, when considering the possibility of a hostile laser attack, the possibility of reflection of energy from the blanket onto another surface of the spacecraft is a possibility, so a highly reflective surface could be disadvantageous under that condition.
In Table I, the .alpha. for 3-5 .mu.m is listed as "&gt;0.20". The value observed varies, depending upon the exact wavelength within the band, but the least value which was observed was 0.20. Values of up to 0.8 may have been observed. Similar comments apply to other values in Table I.
The prior art blankets are effective in thermal control of a spacecraft body, or such portions of the body as may be covered. However, the above-described prior art multilayer thermal blanket is opaque to radio-frequency (RF) signals. For this purpose, RF refers to that portion of the electromagnetic spectrum lying between the UHF band (30 to 300 MHz) and the S band (2 to 4 GHz). If the spacecraft includes antennas which must be used while the spacecraft is exposed to a source of radiant energy, some method for thermal control must be used other than covering with the above-described blanket.